This project studies morphine's action upon the monkey's abilities to detect noxious heat stimuli applied to the face. Macque monkeys were trained to detect small temperature increments (0.4 degrees C, 0.6 degrees C or 1.0 degrees C) from a 46 degrees C baseline. The animals also performed an innocuous thermal cooling task in which they detect a 0.5 degrees C decrease from a 38 degrees C baseline. Additionally the monkeys perform a difficult visual detection task. Morphine sulfate was microinjected in the medullary dorsal horn (0.2 microliters in 2 minutes). Morphine reduces in a dose- (1, 3 or 10 micrograms) and stimulus-dependent manner the monkeys' ability to detect 0.4 degrees C, 0.6 degrees C and 1.0 degrees C. These effects were reversed by the specific opiate antagonist naloxone (0.5 mg/kg/i.m.). Morphine did not affect the detection of a small visual stimulus and the 0.5 degrees C decrement in the innocuous cooling range. These data demonstrate that morphine specifically attenuates noxious heat sensory discrimination when directly applied to the medullary dorsal horn, the earliest central relay transmitting noxious information from the face.